The present invention relates generally to circuitry for instant-start gas-discharge lamp ballasts. More particularly, the present invention relates to circuitry designed to overcome arcing problems associated with current-fed, parallel-resonant inverter topologies used in electronic ballasts.
The instant-start type of fluorescent lamp ballast has the advantage of fast starting. It is cost-effective and is particularly appropriate for long, continuous operation. A current-fed parallel-resonant inverter topology is known in the art as a particularly good solution for this kind of instant start application.
Referring to FIG. 1, an example of a typical topology for conventional dual lamp current-fed parallel-resonant circuits is shown. In this example, it may be understood by one of skill in the art that V_bus is typically provided at the output of a power factor control (PFC) circuit. Capacitors C1 and C2 may be large electrolytic components with C1 equal to C2. Q1 and Q2 are bipolar junction transistors (BJTs) that are used as switching components. D1 and D2 are free-wheeling diodes associated with transistors Q1 and Q2. Resonant capacitor C_Res and the primary winding of the main transformer T_Res_P form the main resonant tank.
In the load circuit 2 shown, two gas discharge lamps L1 and L2 are connected with the secondary winding of the main transformer T_Res_S through ballast capacitors C5 and C6.
Prior to starting, V_Bus charges capacitor C4 through resistive network R3 and R4. When the voltage across C4 reaches the breakdown voltage of the diac 3, diac 3 breaks down and looks like a short circuit so that the voltage on capacitor C4 turns on transistor Q2. After Q2 is turned on, the circuit begins to oscillate and the secondary winding of the main transformer T_Res_S, as well as windings T_Res_base_1, and T_Res_base_2 continue driving Q1 and Q2 such that the inverter reaches a steady state.
However, potential arcing within lamp-holders of instant-start type ballasts is a phenomenon that has been recognized as an undesirable effect to be mitigated. Such ballasts may have ignition voltages double or more that of a preheat type of ballast, and are therefore more conducive generally to potentially damaging arcing. Arcing may occur during re-lamping conditions where a gas discharge lamp is installed or replaced during live application of AC power. This form of arcing is relatively instantaneous and potentially undesirable. Arc detection should generally be suppressed during certain conditions such as normal startup or lamp ignition given the varying needs of gas discharge lamps. On the other hand, sustained arcing that occurs because of improper connections may be far more damaging and must be quickly and efficiently addressed.
It is therefore desirable that an arc protection circuit be designed for use with a fluorescent lamp using an instant-start type ballast.
It is further desirable that the circuit provide inverter shutdown capability for current-fed parallel-resonant inverter topologies with a predetermined time delay measured from lamp ignition and current conduction to prevent false or otherwise undesirable shutdowns.
It is further desirable that the circuit provide auto-restart capability for current-fed parallel-resonant inverter topologies with a predetermined time delay after disabling of the inverter.